This is definitely a flexible recipe. If it suddenly gets a little too thin after adding milk you can always add a bit more powdered sugar to help thicken it again and vice versa. You will get a feel for it as you do it more. I always find when I make smaller batches for little projects, it takes a little playing to get the consistency just right, but thankfully it's pretty hard to mess it up as long as you have a little patience and only add a a little bit of milk or additional sugar at a time while you are trying to get the right consistency. At that point a little goes a long way.